1. Field of the Invention
The present invention relates to a wave plate to be used in an optical pickup unit, and particularly relates to the wave plate which compensates temperature dependence of a phase difference.
2. Background Art
In recent years, there is a need to process a vast amount of various information including characters, images, and the like at a higher speed along with the penetration of the multimedia. For this reason, there is an increasing demand for compact optical disks such as a CD and a DVD that can be read and written at a high speed with storage capacity remarkably larger than that of a conventional magnetic recording media. Optical pickup units are used to read digital signals of sounds, images, and the like written into the optical disks. The units emit laser beams onto optical disks and pick up their reflected light so as to read digital signals from recording media created as pits on surfaces of the optical disks. Wave plates which are used as optical parts of the optical pickup units change a polarizing state of light. Such wave plates include ¼ wave plates which shift a phase difference by 90 degrees so as to convert linear polarized light into circular polarized light or convert circular polarized light into linear polarized light, ½ wave plates which shift a phase difference by 180 degrees so as to rotate a polarization plane of linear polarized light, and the like. FIG. 5 is a diagram showing a constitution of the ½ wave plate. Linear polarized light enters a ½ wave plate 11, and when an angle formed by the incident polarization plane and an optical axis of a board is assumed to be θ, emitted light rotates by 20 with respect to the incident polarization plane so as to become linear polarized light.
The ½ wave plates made of a crystalline material such as crystal include a ½ wave plate which is constituted by laminating two wave plates as well as the ½ wave plate constituted by a single plate. In such a ½ wave plate, two wave plates are laminated so that a phase difference is 180 degrees, and although the cost becomes higher than that of single-plate constitution, ½ wave plate having excellent incident angle dependence can be realized. FIGS. 6(a) and 6(b) are diagrams showing the constitution of the ½ wave plate where two wave plates are laminated. A wave plate A has 1080 degrees of a phase difference and 15 degrees of an optical axis azimuth, a wave plate B has 900 degrees of a phase difference and 105 degrees of an optical axis azimuth. The wave plates A and B are laminated so that an angle formed by the optical axes of the wave plates A and B becomes a right angle, thereby rotating an emitted light polarization plane by 30 degrees.
The temperature dependence of the wave plate is examined. FIG. 7 is a diagram showing the temperature dependence of the phase difference of the wave plate. As shown in FIG. 7, the phase difference of the wave plate is 180 degrees under the condition of the room temperature 25° C., but when the temperature shifts from the room temperature, the phase difference shifts from 180 degrees. When the temperature dependence of the phase difference of the wave plate is high, the phase difference of the emitted light changes due to the temperature, and this phenomenon is not preferable for the optical pickup unit. The present invention solves the above problem, and its object is to provide a wave plate which is constituted by laminating two wave plates and where temperature dependence of a phase difference is small, and an optical pickup unit using the same.